End-Hall ion sources are used in a wide range of industrial applications. They are subject to a variety of heating and maintenance problems. The object of this invention is an end-Hall ion source that is easy to maintain when operated at high power.
Ions are generated by electrons emitted from an electron emitting cathode that is operated at a potential near ground. Ground is defined here as the potential of the surrounding vacuum chamber, which is usually (but not always) the same as earth ground. The electrons are attracted to the anode, which is at a positive voltage relative to ground—from several tens of Volts positive up to several hundreds of Volts positive. As the electrons enter the discharge region enclosed by the anode, they gain sufficient kinetic energy to ionize atoms or molecules of the ionizable working gas. The electrons are prevented from directly reaching the anode by a magnetic field between the internal pole piece and the external pole piece. Because of the magnetic field the electrons follow a long path in the discharge region before reaching the anode, thereby permitting operation at a much lower pressure for the ionizable working gas than would be possible without the magnetic field. Some of the ions generated in the discharge region escape out the open end of this region toward the electron emitting cathode and, together with some of the electrons emitted from this cathode, form a neutralized ion beam. “Neutralized” here refers to nearly equal densities of electrons and ions, not the recombination of the electrons and ions.
There is a reflector between the anode and the internal pole piece that defines the internal end of the discharge region. This reflector is electrically isolated and “floats” at a voltage intermediate of the anode and ground. This intermediate potential avoids the excessive erosion of the reflector that would take place if it were at ground potential, as well as the excessive loss of ionizing electrons if it were at anode potential. This reflector has been called a gas distribution plate or distributor, for its function in distributing the ionizable working gas. It has also been called a reflector, for its role in reflecting and conserving the ionizing electrons. It will be called a “reflector” herein. The ion source is enclosed by the return path for the magnetic field between the internal and external pole pieces. This enclosure also serves to exclude the electrons and ions that exist in the vacuum chamber outside of the ion source. These electrons and ions would otherwise cause damaging and performance-degrading arcs between electrodes inside the ion source. The enclosure also serves to exclude particles which would otherwise be deposited inside the ion source and result in a more rapid coating and degradation of insulators. The magnetic field could be generated by an electromagnet, but is usually generated by a permanent magnet adjacent to, or incorporated with, the internal pole piece.
A variety of operating and maintenance problems are encountered with these ion sources. Many of the problems have to do with heating. The energy input to the ion source is mostly from the discharge energy, that is, the current to the anode times the potential of the anode. Some additional energy is required to generate electrons, either the heating power for a hot-filament, cathode or the discharge power in a hollow-cathode type of cathode. Excessive heating can demagnetize the permanent magnet. It can also cause melting of the anode or reflector. Various cooling techniques have been used to avoid the problems caused by excessive heating. But these cooling techniques have often caused new problems. There have been cooling lines (carrying liquid coolant) that must be opened to perform maintenance, then re-connected to resume operation, with the possibility of cooling-line leaks in the vacuum chamber from the opening and re-connecting of these lines. Cooling the anode directly requires voltage isolation in the cooling lines, with the added problems of degradation of the insulator used and the enhanced erosion in the cooling lines caused by the applied voltage. Indirect cooling of the anode involves the conduction of heat through thin layers of insulation which, depending on the insulator, are easily broken or penetrated. It can also be difficult to maintain reliable heat transfer through thin layers of insulators due to poor thermal conductivity or poor thermal contact. As an additional source of problems, maintenance by the ion-source user can sometimes be carried out without regard for the manufacturer's instructions.